Sorption has been found to be superior to other techniques for cleaning up water pollution by reason of simplicity of design, ease of operation, speed of action and insensitivity to toxic substances. There is a great demand for hydrophobic oleophilic sorbents. Key requirements of such sorbents are high selectivity, high capacity, rapid uptake, good buoyancy and long life. They have to be available in tonnage quantities at economical cost. They should have a particle size, shape and mechanical strength suitable for practical use. As the world addresses a growing list of environmental problems, the qualities of renewability, non-toxicity, biodegradability and biocompatibility of sorbents for pollution control are important.
Various materials have in the past been found useful for sorbing oils and hydrocarbons, such as the activated carbons, synthetic organic sorbents, mineral-based sorbents, coating-based sorbents, peat moss and others. These are all relatively expensive, as a result of the raw material cost, the processing cost and the packaging cost, and therefore have limited application on an industrial scale. Other common disadvantages of these known materials are briefly summarized as follows: some have low buoyancy or even sink in water; some have low oil absorption capacity; some have slow sorption; some lose the sorbed oil very easily; some lose oil absorption ability immediately after contacting water; some exhibit the same stickiness of the sorbed oil after sorption; some do not work effectively under many environmental conditions typically encountered, such as at low temperature; some have to use chemicals for modification which are themselves environmental contaminants; some are toxic; some are too light to be spread effectively in actual field use; some have limited source supply of raw material; and some are not biodegradable.
Natural plant and agricultural products and residues have long been used for sorption of oils and hydrocarbons. These materials have the advantages of being inexpensive, readily available and easily supplied in bulk, granules, mats, pads, nonwoven sheets and used in continuous-working devices. For example, untreated sawdust has been used to sorb oil, and untreated straws and feathers to clean up oil spilled on water. However, it has always been considered that the natural plant and agricultural materials sorb water too rapidly, thereby sinking, to be useful for oil and hydrocarbon control on water surface.
Because of the economic attractiveness and the environmental benefits of natural materials and biowastes as the raw materials for hydrophobic products and sorbents, a number of attempts have been made to make the naturally hydrophilic plant and agricultural materials hydrophobic and oleophilic. The modification efforts are mainly in three categories: coating, reacting and heating.
The coating process modifies the raw material by adding hydrophobic reagents or polymers to get a hydrophobic surface. Examples are shown in U.S. Pat. Nos. 4,519,918, 4,925,343, 5,021,390, 5,492,881 and 5,891,937. The additives add to the manufacturing cost of the products and can be a source of environmental contamination.
The reacting process modifies the raw material by chemical reactions. Examples are shown in U.S. Pat. Nos. 2,358,808, 3,770,575, 3,874,849 and 4,605,640. Again, the reaction agents add to the manufacturing cost of the products and may be the source of environmental contamination.
The heating process modifies the raw materials by thermochemical reactions. Hydrophobic substances are produced from the components of the raw materials themselves during the heating process. The manufacturing process is simple, low cost and has no toxic material involved. Prior heat treatments of lignocellulose materials, mainly comprise processes of thermocondensation, torrefaction and carbonization at different temperature levels. Thermocondensation is the thermochemical degradation reaction of lignocellulose material at temperature between 200° C. and 280° C. An example of such a process is shown in U.S. Pat. No. 4,954,620. Torrefaction consists in briefly exposing the lignocellulose material to a temperature between 270° C. and 300° C. while in contact with the air and under the influence of direct heat in order to cause incomplete carbonization. Examples of such a process are shown in FR 839 732 and 872 164, DE 2 802 213 and EP 0 073 714. Carbonization takes place at higher temperatures, preferably about 450° C., in order to provide maximum elimination of the tars which are generated by destruction of the ligocellulose material.
U.S. Pat. No. 4,553,978 (Yvan) discloses a process for converting ligneous matter of vegetable origin by torrefaction in a neutral atmosphere at a temperature of between 200° and 280° C., and preferably between 240° and 260° C., for a duration of 30 minutes to 5 hours.
U.S. Pat. No. 4,753,917 (Grenthe) discloses a hydrophobic sorbent which is prepared by subjecting water-containing, fibrous cellulosic products, particularly sulphite reject fibers, to rapid heating to cause expansion of the fibers through gasification of the water therein. Preferred heating is operated in a stream of high temperature air from about 500° F. to 700° F. for several minutes. After the rapid heating to expand the fibers, a thin coating of waxy material is further applied on the surface thereof.
U.S. Pat. No. 4,954,620 (Bourgeois) discloses thermocondensed lignocellulose material which has a hemicellulose content of less than 2% and a calorific value which is about 20% greater than that of the starting material is obtained by isothermochemical treatment between 220° C. and 280° C. for a period of thirty minutes using crossed flows of treated material and of oxygen-free hot gases.
U.S. Pat. No. 5,110,785 (Reed) discloses a novel composition of matter which is prepared by subjecting at least one woodlike particle such as dry pine sawdust, to selectively controlled thermolytic heating above about 280° C., but not above about 380° C., and preferably between 300° C. and 360° C. for about ten minutes to cause the hemicellulose to be converted to an oil-like oleophilic and hydrophobic substance. The heating is carried out in a rotary oven. Air circulation or the type of atmosphere is not mentioned in the heating system.
U.S. Pat. No. 5,585,319 (Saitoh) discloses a process for preparing an oil sorbent by heating lignocellulose at a temperature of 250° C. to 450° C. for 5 to 100 minutes in a rotary oven with no air inlet but an outlet which permits escape of pyrolignous acid and pyrolignous gas.
JP 62,050,393A2 (Fumiaki) discloses a heat treatment of coal by heating at a temperature of 180° C. to 300° C., with an inert gas having an oxygen content of at least 10 volume %, a hot gas containing at least 10 volume % steam, or a 100% steam for preventing the burning of coal or explosion. By heating the coal above 180° C., the internal moisture of the coal is decreased and the oxygen-containing hydrophilic groups, such as a phenol group and a carboxylic group, are thermochemically decomposed to be eliminated so that the coal becomes hydrophobic, and the hygroscopicity is decreased.
JP 11,009,992A2 (Tsutomu) discloses a gas absorbent manufactured from residues of coffee beans from which coffee components are extracted by boiling water. Residues are heat treated under oxidizing atmosphere at temperature in the range of 300-450° C.
FR 953,004 and Swiss 228,877 disclose a torrefaction operation which takes place from 250° C. to 350° C. and from 250° C. to 300° C. respectively, without any precision relative to the atmosphere in which the operation is carried out, from which it is concluded that the atmosphere is of no particular importance and that, in practice, the operation is carried out in a normal ambient atmosphere.
Generally, high temperatures are supplied for a short time under non-oxidizing gas medium or wet steam atmosphere in the prior art heating processes. When an air medium is used, it is simply because air is the most economical and readily available atmosphere, not for the purpose of oxidation.
Oxidation or ozonation treatment is well known in industrial applications, such as for pulp bleaching in paper industry and for fiber activating in graft polymerization. The treatment is usually carried out in high concentration of oxidant at low temperatures in aqueous environment. The products are usually hydrophilic. U.S. Pat. No. 4,459,174 (Papageorges) discloses a process for the delignification and bleaching of chemical and semi-chemical cellulosic pulps in which the pulp is subjected to a treatment with oxygen in an alkaline medium at a temperature of between 353° and 423° K (80° and 150° C.), and a subsequent treatment with peroxide at a basic pH. U.S. Pat. No. 4,120,747 (Sarge, III) discloses a soft, hydrophilic absorbent, bulky-paper web formed by thermomechanically defibrated pulp from wood chips which have been soaked in chemical solutions prior to defibrating and then treated with ozone at a temperature of from 40° to about 55° after defibrating. U.S. Pat. No. 6,020,278 (Gatenholm) discloses a method in graft polymerization for the production of highly hydrophilic absorbent hybrid fibers by ozoning at a temperature of in the region of 15-60° C. during a period of time which lasts up to 90 minutes, preferably in the form of steam. U.S. Pat. No. 5,549,789 (Atalla) discloses a method for wet oxidative degradation of lignin and polysaccharide fragments dissolved during polyoxometalate delignification or bleaching of wood fibers or wood pulp to volatile organic compounds and water. U.S. Pat. No. 5,346,549 (Johnson) discloses a method of producing environmentally stable formed bodies useful as building material comprising papermill sludge, ash and water treated with an oxidant and exposed to electromagnetic energy, preferably ultraviolet light, at ambient temperature and without the use of a drying oven. WO 88/09622 (Olson) discloses a method for reducing the amount of oxalic acid and/or sulfites in a sugar beet with an oxidizing compound such as hydrogen peroxide at about 30° to 60° C.
With respect to making hydrophilic products, some efforts have been made to alter hydrophobic surfaces into hydrophilic ones by oxidation at low temperatures. For example, U.S. Pat. No. 5,369,012 discloses a method of producing an organic polymer membrane that is made hydrophilic by exposing a hydrophobic surface of the article to atomic oxygen or hydroxyl radicals at a temperature below 100° C., preferably below 40° C., to form a uniform hydrophilic surface layer of hydrophilic hydroxyl groups. Some efforts have also been made by heating with or without crosslinking agent. For example, U.S. Pat. No. 5,137,537 (Herron) and U.S. Pat. No. 5,873,979 (Naieni) disclose a hydrophilic absorbent structure containing individualized, polycarboxylic acid crosslinked cellulosic fibers by heating uncrosslinked cellulosic fibers with an amount of C2-C9 polycarboxylic acid crosslinking agent in an intrafiber ester crosslink bond form. The heating is carried out for a period ranging from 5 seconds to 2 hours at an air temperature of 120° C. to 280° C. to remove any remaining moisture content and cause crosslinking to occur. Preferably, the crosslinking agent is citric acid. U.S. Pat. No. 5,709,774 (Naieni) discloses a method of preparing heat-treated-in-air high cellulosic fibers, for use in absorbent structures, which are free of moieties from crosslinking agents by fluffing and heating in air at atmospheric pressure at a temperature ranging from 120° C. to 280° C. for at least 5 seconds.